1. Field of the Invention
This invention relates to methods for modifying milkweed oil with cinnamic acids to form ultraviolet (UV)-A and (UV)-B absorbing esters.
2. Description of the Prior Art
Health hazards associated with exposure to the sun are well established. The short term effect of excessive exposure to sunlight is erythema, commonly referred to as sunburn. Sunburn is primarily the result of UVB radiation having a wavelength of from about 290 nm to about 320 nm. Long term effects of exposure to sunlight include skin cancer (melanoma) and premature aging of the skin (including wrinkling, loss of elasticity, and pigment changes). These effects are predominantly caused by UVA radiation having a wavelength of from about 320 nm to about 400 nm. Public awareness of the dangers of sun exposure has stimulated the market for personal care products containing sunscreens.
Sunscreens function either as ultraviolet (UV) filters or UV blocks. UV blocks, such as TiO2 and ZnO, as well as derivatives of other metal-oxides, form a physical barrier that scatters UV light (Fairhurst et al., “Particulate Sun Blocks: General Principles”, Sunscreens: Development, Evaluation, and Regulatory Aspects, 2nd Edn, pp. 313-352, 1997). These UV blocks offer the most comprehensive sunscreen protection, blocking the full spectrum of UVA (400-320 nm) and UVB (320-290 nm) light. As a result of the particulate nature of these formulations, they often leave a noticeable residue when applied to the skin, which is cosmetically unacceptable to the consumer. The most commonly used sunscreens are UV filters, which are typically organic compounds incorporated at levels of about 2-15% into topical formulations (N. A. Shaath, “Evolution of Modern Sunscreen Chemicals”, Ibid, pp. 3-33, 1997), (N. A. Shaath, “Quality Control of Sunscreens”, Ibid, pp. 657-676, 1997). A disadvantage of UV filters is that each organic compound has a limited range of maximum UV absorptivity, rendering each reagent better suited for either UVA protection or UVB protection but not both. The advantage of the UV filtering molecules, however, is that they can be engineered to provide sunscreens with desirable physical appearance, solubility, and water resistant properties (N. A. Shaath, “Quality Control of Sunscreens”, Ibid, pp. 657-676, 1997).
Although no longer used today, benzyl cinnamate formulated as an emulsion with benzyl salicylate, was used as a sunscreen as early as 1928 (N. A. Shaath, “Evolution of Modern Sunscreen Chemicals”, Ibid, pp. 3-33, 1997). Today, cinnamic acid derivatives are the most widely used UVB absorbing chemicals in sunscreen formulations, with four derivatives approved for use in the United States and 17 approved for use in Europe (N. A. Shaath, “Evolution of Modern Sunscreen Chemicals”, Ibid, pp. 3-33, 1997). The unsaturated C═C bond adjacent to the aromatic ring in cinnamates allows for a continuous, conjugated p-system throughout the molecule. An electron can be delocalized throughout the p-system by photo-excitation with energy corresponding to about 305 nm. Most common cinnamic acids and short chain esters are water soluble, limiting their usefulness as waterproof sunscreens. Cinnamic acid derivatives, therefore, have been designed with long chain hydrocarbons (i.e. octyl-p-methoxy cinnamate), which renders them water-insoluble and suitable for waterproof sunscreens. The —OCH3 group of octyl-p-methoxy cinnamate acts as an electron-releasing group to improve the electron excitation process (N. A. Shaath, “Evolution of Modern Sunscreen Chemicals”, Ibid, pp. 3-33, 1997).
There is currently a growing interest in modifying fats and oils to form structured lipids with specific properties for nutritional and pharmaceutical applications. Recent reviews have outlined the strategies for synthesizing tailor-made fats and oils and their desired properties (Willis et al., “Lipid Modification Strategies in the Production of Nutritionally Functional Fats and Oils”, Crit. Rev. Food Sci. Nutr. 38:639-674, 1998), (F. D. Gunstone, “Movements Towards Tailor-Made Fats”, Prog. Lipid. Res. 37:277-305, 1998). These strategies have included blending, distillation, fractionation, hydrogenation, interesterification with chemical catalysts, and more recently interesterification with biocatalysts. Chemical interesterifications of triacylglycerols for industrial applications are typically performed using inorganic catalysts at elevated temperatures (200-250° C.) (N. N. Gandhi, “Applications of Lipase”, J. Am. Oil Chem. Soc. 74:621-633, 1997). Enzymatic interesterifications, however, offer the advantages of milder reaction conditions, a wider variety of synthetic substrates, and regioselective specificity towards the acyl groups of the triglycerols (Schmid et al., “Lipases: Interfacial Enzymes with attractive Applications”, Angew. Chem. Int. Ed. 37:1608-1633, 1998).
Compton et al. (U.S. Pat. No. 6,346,236 hereby incorporated by reference) teaches the formation of sunscreens from vegetable oil and plant phenols by use of a lipase catalyzed transesterification reaction to yield novel ferulyl-substituted or coumaryl-substituted acylglycerols.
Apart from the aforementioned efforts to develop improved sunscreen agents, there has been a resurgence in recent years to cultivate the common milkweed (Asclepias syriaca L.) as an alternative crop (Knudsen, H. D. et al., 1993, the Milkweed Business, pp. 42-428, In: J. Janick and J. E. Simon (eds), New Crops, Wiley, N.Y.), with the primary focus on marketing the floss as a substitute for waterfowl down. A byproduct of floss production is the seed which is rich in milkweed oil composed of 45-50% linoleic acid.